The present invention relates to anchors for securing screws in materials such as concrete and more particularly to electrically insulated threaded fastener anchors.
It is often desirable to attach an article or device to a structure that is formed of a hard material such as concrete. To this end, there are various anchor arrangements made for insertion in pre-drilled holes that allow use of screw type fasteners. In some circumstances, however, it is desirable that the anchor be electrically insulated. For example, the metal rails of railways are often used to transmit electrical signals. It thus becomes necessary that the rails be electrically insulated from one another to avoid the potential of disrupting such signals.
Insulated threaded fastener anchors have been developed in which an electrically inslulative material, usually plastic, is used between the screw and substrate. In some forms of anchors, a polymer coating is disposed about the external surface of a metal anchor. The coating will function to insulate the encased anchor and screw, if the coating remains intact. However, it is not unusual that the coatings are scratched or chipped during installation of the anchor and the insulative quality is thereby compromised.
In another anchor form, a thin metal sleeve is provided around a plastic screw receiving insert that insulates the substrate from the screw. This type of anchor is more serviceable to provide insulating properties, but the holding strength desired to secure the screw is compromised.
Lateral stress applied against the screw will tend to work the insert and weaken the ability of the anchor to hold the screw against unintended withdrawal. Too frequently, the flexing anchor will break or crack the adjacent brittle substrate media.
Further, the typical prior forms of threaded anchors will typically include ribs spaced along the anchor sleeves for use to resist pulling forces exerted by the screw. Tension applied by tightening the screw within the anchor will result in radial outward forces being applied at stress risers created by the ribs. Thus, tension applied by the screw, or lateral shock loading can produce excessive forces at the stress concentrated areas, resulting in cracking or breakage of the substrate materials.
One of many examples of anchor sleeves that do not include an axially smooth outer surface is shown in U.S. Pat. No. 5,641,256 to Gundy. However, Gundy shows a sleeve that has both longitudinal and transverse ribs which are used to lock the sleeve in a casting. Gundy recognizes the problem of stress risers, or xe2x80x9cfragile zonesxe2x80x9d created by the traditional ribbing on anchor members. The solution attempted by Gundy is to add more ridges extending longitudinally along the body of the anchor. The intersecting ridges, however, do not uniformly eliminate all stress risers, since annular ridges remain along the sleeve between the longitudinal ribs, and since the longitudinal ribs themselves create irregularities along the sleeve surface.